Geophysical apparatus and method



Aug. 4, 1942. w. R, WELTY GEOPHYSICAL APPARATUS AND METHOD Filed May 15,1940 2 Sheets-Sheet 1 -4 4 w. R. WELTY 2,291,779

GEOPHYSICAL APPARATUS AND METHOD Filed May 15, 1940 2 Sheets-Sheet 2Patented Aug. 4, i942 GEOPHYSICAL mnns'rus AND METHOD William RobertsonWelly, San Antonio, Tex, assignor to Olive 8. Petty, San Antonio, Tex.

Application May 13, 1940, Serial No. 384,870

2 Claims. (Cl. 177-352).

This invention relates to seismic apparatus and methods such as may beemployed in geological surveying. It lsthe principal object of theinvention to improve and simplify the equipment commonly used for thispurpose and to alter the method of use thereof so as to effect a recordof seismic impulses which may be more readily interpreted.

It has heretofore been known that reasonably accurate mapping ofgeological formations can be eiiected by creating seismic waves at apoint or points near the earth's surface and determining at one or morepoints remote therefrom the lapse of time required to enable the wavesto reach such remote points. The customary procedure in such surveyingis to fire a charge of explosive at a suitable distance on or at asuitable distance below the surface of the earth at a position commonlyreferred to as a shot point," and to detect or receive the resultingdirect, reflected, and refracted seismic waves at remote points wherethey are converted into electrical energy oi varying voltage. Suitableapparatus is then employed to record the fluctuations of this electricalenergy in permanent form for study.

Conventional equipment for recording the seismic waves includes at thereceiving point a device variously called detector, seismograph,seismometer, geophone, or the like. Such an instrument may take as manyforms as it has.

names, and in general comprises a relatively steady mass and a partmovable with the earth,

these partssupporting electrical elements relatively movable thereby,whereby electric voltages are generated or varied in accordance withvariation in the strengths of'the respective energizingwaves. Theelectrical output from such a seismometer is customarily amplified withsuitable equipment and fed into a recorder which includes a galvanometerhaving a moving element whose motion, by suitable optical means, isrecorded on a photographic sheet caused to move by clockwork at asubstantially fixed rate and also to have recorded thereon deilnitetime'intervals from an appropriate timer.

It has heretofore been proposed to materially reduce the magnitude ofthe energy derived from waves propagated generally horizontally bysuitable spacing of a plurality of seismometers. For example, if twoseismometers are spaced, as measured in the direction of propagation ofsuch waves, by one-half of, the wave length thereof. the combined energyoutput of the two seisstantially cancel. Such spacing will notmateriallyaiiect, however, the energy derived from waves which penetrateto a considerable depth and are reflected and/or refracted upwardly,since such waves ordinarily approach the surface in a more nearlyvertical direction. In other words, by employing a plurality ofseismometers and spacing them in the direction of propagation of thewaves which travel in the upper crust,

so that their combined energy output will approach zero, the energyderived from such waves is thereby considerably reduced in magnitudewith respect to the energy derived from waves reflected and retractedfrom considerable depths.

It is also proposed to reduce materially the response to random energy,and for this purpose the seismometers need not be spaced in thedirection of propagation of the generally horizontal waves. Thus, it ishighly unlikely in a system employing a number of seismometers thatrandom energy will be received at the several seismometers in phase,regardless of the arrangement of the seismometers with respect to eachother or with respect to the source of seismic impulses. Actually,random energy may affect one only of a group of seismometers, noappreciable response being received at other seismometers of the group,in which event the effect of the random energy will be negligible.

It is customary to employ a considerable number of seismometers, forexample, eight or nine, located at diflerent distances from the shotpoint," and to separately amplify and record the energy output of theseseismometers so that the time of arrival of the seismic impulses at thevarious points may bedetermined. In the practice of the presentinvention each of these seismometers may be replaced with a group ofseismometers, the seismometers in each group preferably being spaced ashereinbefore indicated and having their energy output combined.

The use of multiple cables or many separate cables for connecting theseveral seismometers with suitable amplifying and recording apparatus iscommon in existing practice. Thus the signal energy received at theseismometer is quite feeble, and in order to establish a favormometerswill be 180 out of phase andwill subtion, with resultant multiplicationof the number oi conductors. when, as is customary, a considerablenumber of seismometers is employed,

the handling and laying of the cables is a matter or considerabledifliculty and the cables are expensive.

It is therefore a further object of the instant invention to materiallyreduce the number of conductors required and to facilitate theconnection oi the same to the respective seismometers.

More specifically, it is an object of the invention to provide a systemfor seismic surveying in which only one cable need be employed, the

cable being inductively, directly, capacitatively,

or otherwise coupled with the seismometers in 1 series, for ple bylooping the cable around each seismometer in succession. Such anarrangement not only facilitates coupling ofthe seismometers with theamplifying and recording equipment, but permits the use of seismometerswhich can be much more easily sealed to exclude dirt and moisture, nodirect electrical connection between the exterior and the interior ofting a modulated waveform electrical signal having a distinctive carrierlreuuency, and for thereafter isolating the several carrier frequencies,and separately detecting and recording the modulation energy thereof.

Fiu'ther objects and features of the invention will be apparent irom thefollowing description taken in connection with the accompanyingdrawings, in winch Figure l is a diagrammatic view illustrating 45 thesecondary winding 25 and is adjusted to the grouping of the seismometersand showin the circuits for isolating, detecting, and amplitying thetransmitted signals;

Figure 2 is a diagrammatic representation of the seismometers in onegroup, showing the in ductive coupling between one of the seismometersand the transmission line, but omitting other elements of thetransnutting circuit; and

Figure 3 represents a form of transmitting circult suitable for use inthe transmitting seismoineters shown in Figures 1 and 2.

In order to facilitate an understanding of the invention, reference ismade herein to the embcdiment thereof illustrated in the drawings andspecific language is employed to describe the same. It will neverthelessbeunderstood that no limitation of the invention is thereby intended,and that various other modifications and alterations of the illustratedand described arrangements are contemplated such as embody theprinciples of the invention and fall within the scope of the claimsappended hereto.

Figure l of the drawings discloses an arrangement of seismometers inthree groups, four seismometers being employed in each group, but itwill be understood that a larger number of groups, for example eight ornine, can be successfully employed and that the number of seismometersin any group may be two or more.

Seismometers designated by the numeral ll may contain nothing more thanthe usual sta- 55 the cathode tionary and movable plates constituting acondenser which is variable in response to received impulses. The platesof the seismometcrs ill in each group are connected in parallel to simi-5 lar movable and fixed plates in the associated seismometer l2, and thelatter contains, in addition, means for transmitting a wave-form sig-Jnal which is modulated in accordance with the vibratory movement of theseveral movable plates in that seismometer group, as is hereinafter morefully described.

In the form of theinventlon shown herein,-

the modulated wave-form signal is transmitted to a suitable insulatedconductor l5, this conductor being looped around each of thetransmitting seismometers l2 so as to place the conductor in inductiverelation with a transmitting coil which is preferably disposed withinthe seismometers l2. One end of the conductor I5 is grounded asindicated at l6, and the other end feeds the signal to suitablefiltering, detecting, amplifying, and recording devices which may bearranged as shown in Figure 1.

Thus, a plurality of filter circuits, are shown,

the number corresponding to the number of transmitting seismometers l2.For convenience, in describing the filter circuits and other associatedelements which are similarly duplicated, the same numerals are employedwith suitable 39 subscripts, reference being made hereinafter forconvenience only to the numerals. Thus, each filter circuit may includea variable condenser 20 and an inductive winding M in series between theconductor l5 and the ground connection, indicated at 22. Each filtercircuit is adjusted to resonate at a frequency slightly higher than thecarrier frequency of one only of the transmitting seismometers. Thewinding 2i constitutes the primary of a transformer, the

40 secondary winding, indicated at 25, being connected at one end to thegrid 26 of a thermionic valve 2? and at the other and through a source28 oi grid bias voltage to the cathode 29. A variable condenser 23 isarranged in parallel with suitable adjustment or the grid bias voltageit or otherwise. The anode 53b of the valve ill delivers the valveoutput to the primary winding st of a transformer 32, the circuit beingcompleted through a source 5556 of anode voltage to The secondarywinding of the transformer 32 is connected through a suit- -ablethermionic valve amplifier til to a galvanometer or recording device 38,for example, a multiple string galvanometer of the recording typeaffording one string for each of the filter circuits, so that aseparaterecord may be made of the signal transmitted by each of thetransmitting seismometers.

The function of the seismometers is further illustrated in Figure 2 ofthe drawings, representing certain of the essential elements of theseismometers of one group of four. Thus, each of the seismometers l0 maycontain a movable plate 43, having associated therewith suitable 7oelastic suspension means represented by the spring ll, plate 43 beingsupported in close proximity to a fixed plate 45, so that the capacitybetween the plates varies as a function of the seismic impulses whichare applied to the seismometers. The plates in the seismometers l0,

as well as the corresponding plates in the seismometers 92, areconnected in parallel by means of conductors H which are preferably ofthe coaxial cable type, so as to provide, in efiect, four variablecondensers in parallel. The transmitting circuit, the essential elementsof which are shown in Figure 3, is preferably housed in the seismometerit, but to avoid confusion only the transmitting winding, indicated atdd, is shown in Figure 2. This winding may be placed within thegenerally cylindrical wall of the seismometer, or may be wound so as tolie on the inner surface of the wall or may otherwise be convenientlydisposed for inductive relation with the secondary winding E3, formed bylooping the conductor 95 about the exterior of the seismometer. Unlessthe winding dd is disposed externally of the seismometer casing, it isof course necessary to form the wall of the seismometer of materialwhich will not shield the winding 68 from the winding 59 or otherwiseimpair the inductive relation therebetween. For example, the casing maybe made of Bakelite or other suitable non-metallic transmitting circuitembodied in the seismometer 12, the fixed plates 45 being connected toone end of the winding 68, and the movable plates 63 being connectedthrough a resistance 52 to the opposite end of the winding 48 and thencethrough a source of grid bias potential 53 through the grid 56 of athermionic valve 55. A source 58 01 anode voltage is connected at oneend to the anode 59 of the valve 55 and at its other end to the fixedplates 55. A by-pass condenser 60 is frequencies are detected,amplified, and delivered to the galvanometer or recording device.

In order to eifect substantial cancellation of energy derived by theseismometers included in any one group from waves propagated generallyhorizontally, it is of course not necessary that the seismometers bearranged in alignment with the direction of propagation oi such waves.It

. is only necessary that the spacing between the seismometer-s, asmeasured in such direction, is the required fraction of the wave-lengthof such waves. ermore, accuracy of spacing is not required since theestablishment of a substantial phase erence in the energy delivered bythe several seismometers of a group, as regards-waves travelling in theupper crust, is found to be very helpful, particularly since the energyderived from the deeply penetrating reflected or retracted waves by theseveral seismometers of a group is usually in phase or nearly so, sothat the total response from such waves is increased.

It will be appreciated that signals other than those directlyrepresentative of the seismic impulses y trtted over the singleconducting path. Thus a counicating circuit is often employed betweenthe; shot point" and the recordingstation for the purpose of trans-,mitting a signal at the instant of firing the charge, and to establishtelephonic communication between the two points. Such an arrangement canreadily be incorporated in the present system by the allocation to thetime break signal of a separate carrier frequency, the latter beingmodulated in any convenient manner and isolated and detected at therecording station.

It will also be understood that while the inductive coupling describedherein between the several seismometers and the transmission line ispreferred, other types of coupling may be emshunted across the source 58to aiiorda path for connection so as to aflford the necessary capacihighfrequency current between the anode and one end of the winding 48. Thewinding 48 is provided with an intermediate tap 6|, which is connectedto the cathode 62 of the valve 55. The output winding formed by loopingthe conductor i5 about the seismometer I2 is shown in inductive relationwith the winding 48.

It will be appreciated that the circuit shown in Figure 3 will oscillateat a radio frequency which a is dependent upon the characteristics ofthe various elements of the circuit. The inductances of the two parts ofthe winding 48, the value of the resistance 52, condenser 60, and otherelements of the circuit are appropriately selected to provide aconvenient resonance frequency when the that these constants are soadjusted that the frequency of the oscillating circuits embodied in theseveral seismometers I2 diiler substantially. When seismic impulses areapplied to the scismometers ill and I2 of any one group the capacitiesof the condensers constituted by the fixed and movable plates 63 and 45of these seismometers will fluctuate rapidly, with the result that thefrequency of oscillation of the associated circuit is correspondinglymodified. Frequency modulated oscillating energy is thus applied to thewinding 48, and is delivered through the winding 50 and the conductor IEto the circuits shown in Figure 1. There the signal is filtered ashereinbefore described, and the modulation ployed. For example, acapacitive coupling may be established by disposing a lead wire fromeach seismometer in close proximity to the transmission line, bytwisting the lead wire about the transmission line, or otherwisearranging the tive effect. Obviously a direct connection can be effectedbetween each of the seismometers and the transmission line, and a returntransmission line can be employed where the ground resist- 50anceistoohigh.

-Frequency modulation ofiers certain advantages over amplitudemodulation, particularly in relative freedom from the efiects ofextraneous electrical disturbances, but it will be appreciated that theinvention is not necessarily limited plates 53 are in repose, and itwill be understood 6 in this respect and that the use of other types ofcommunication systems are contemplated. Again, while the employment ofcapacitive seismometers is preferred, various other types, such forexample as magnetic seismometers, may often be employed with equallygood results. Thus, while a capacitive seismometer furnishes its ownmodulating and oscillating system, certain features of the invention maybe practiced with the use of seismometers of widely varying types by theaddition of a modulator and oscillator circuit. Numerous furtheralterations in circuit details will be apparent to one skilled in theart to which the invention relates.

Having thus described the invention, what is claimed as new and desiredto be soured by letters Patent is:

1. In apparatus for use in seismic surveying, the combination with aplurality of seismometers, each including a casing, means within saidcasing ,ior converting seismic impulses to electrical wave iorm signalenergy and for establishing a field varying with said energy externallyof said casing, amplifying and recording apparatus, and

a. conductor delivering energy to saidlast named means. said conductorbeing looped about the several casings successively tor energizationsolely by the several fields. e I

2. In apparatus for use in seismic surveying, the combination with aplurality of seismometers, each including a casing having therein meansexternally of said casina, a conductor looped successively around each'casina .ior energization solely by the several fields, means fed bysaid conductor tor isolating the several carrier waves,

whereby the'modulations thereof may be sepa- 10 rately detected,amplified. and recorded.

, WILLIAM ROBERTSON wnn'rY.

